1. Field of the Invention
The present invention relates to amplifiers having at least three stages and the output stage of which is formed by a MOS transistor. The present invention relates, for example, to amplifiers used to amplify audio signals.
2. Description of the Related Art
FIG. 1 shows an example of an architecture of an amplifier including first 11, second 16, and third 18 stages. The first stage 11 has two input terminals 12, 13 across which a differential voltage VDIFF is applied. Output 14 of first stage 11 is applied to an input 15 of second stage 16. Output 17 of second stage 16 is applied to the gate of a MOS output transistor 18 which forms the third stage of amplifier 10. As an example, output MOS transistor 18 is of type P. The source of output transistor 18 is connected to a first voltage source Vdd. The drain of output transistor 18 is connected to a second voltage source, for example, ground GND, via a D.C. bias current source 19. Drain current ID1, voltage VDS1 between the drain and the source, and voltage VGS1 between the gate and the source have also been indicated for output MOS transistor 18.
A first capacitor 22 connects the drain of output transistor 18 to input 15 of second stage 16. A second capacitor 23 connects the drain of output transistor 18 to output 17 of second stage 16.
An output terminal 24 of amplifier 10 corresponds to the drain of output transistor 18.
A load, including for example a resistor 25 coupled in parallel with a capacitor 26, is connected between output terminal 24 and ground GND with an interposed coupling capacitor 27.
FIG. 2 illustrates, in a logarithmic representation, the simplified shapes of curve 30 of gain G and of curve 31 of phase xcfx86 of the open-loop response of amplifier 10 of FIG. 1 according to frequency.
As appears on curve 30, gain G exhibits three cut-off frequencies f1, f2, and f3. From the low frequencies to first cut-off frequency f1, the gain is substantially constant and equal to a value G0. Between first f1 and second f2 cut-off frequencies, gain G exhibits an attenuation of approximately xe2x88x9220 dB/decade. Between the second f2 and third f3 cut-off frequencies, gain G exhibits an attenuation of approximately xe2x88x9240 dB/decade. Finally, beyond third cut-off frequency f3, gain G exhibits an attenuation of approximately xe2x88x9260 dB/decade. Cut-off frequencies f1, f2, and f3 are respectively associated with first 11, second 16, and third 18 stages.
Phase xcfx86 of amplifier 10 which corresponds to the phase-shift between the signal at output terminal 24 and differential voltage VDIFF is on the order of 180xc2x0 at low frequencies and decreases by 90xc2x0 in the vicinity of each cut-off frequency.
Cut-off frequencies f2 and f3 are given by the following expressions:
f2=gm2*gm3*(RL//rds)/[1+gm3*(RL//rds)]*2xcfx80* C2xe2x80x83xe2x80x83(1)
f3=gm3/(2xcfx80*CL)xe2x80x83xe2x80x83(2)
where gm2 is the transconductance of second stage 16, gm3 is the transconductance of third stage 18, rds is the value of the equivalent resistance of output transistor 18 between the drain and the source, RL is the value of resistor 25 of the load, C2 is the capacitance of capacitor 23, and CL is the capacitance of capacitor 26 of the load. Expression rds//RL corresponds to the value of the resistance equivalent to resistances RL and rds placed in parallel and is equal to rds*RL/(rds+RL).
In equation (1), when term (gm3*(RL//rds) is very large as compared to one, the expression of cut-off frequency f2 simplifies and the following expression is obtained:
f2=gm2/(2xcfx80*C2)xe2x80x83xe2x80x83(3)
A frequency, called the gain-bandwidth product frequency PGB, which corresponds to the frequency at which the gain is equal to one (that is, equal to zero in decibel) is usually defined. The expression of frequency PGB is the following:
PGB=gm1*gm2gm3RL*Req/2xcfx80*C1*(gm2*gm3*RL* Req+1)xe2x80x83xe2x80x83(4)
where gm1 is the transconductance of the first stage, C1 is the capacitance of capacitor 22, and Req is the value of the equivalent resistance seen from the output of second stage 16 of amplifier 10. In the case where term gm2*gm3*Req*RL is very large as compared to one, the expression of frequency PGB simplifies, which then provides:
PGB=gm1/(2xcfx80*C1)xe2x80x83xe2x80x83(5)
For some values of differential voltage VDIFF, amplifier 10 may have a so-called unsteady operation. An unsteadiness of amplifier 10 may translate as the development of parasitic oscillations on the output signal.
A condition to ensure the steadiness of this type of amplifier is that phase xcfx86 at frequency PGB, also called the phase margin Mxcfx86, is positive and, usually, greater than 30 degrees. For this purpose, it is desired to obtain cut-off frequencies such that cut-off frequency f3 is greater than cut-off frequency f2, usually, at least twice as large, and that cut-off frequency f2 is greater than frequency PGB, usually at least twice as large. This enables ensuring the obtaining of a positive phase margin Mxcfx86.
FIG. 3 shows curves 35A, 35B representative of current ID1 of the drain of output transistor 18 according to the opposite of voltage VDS1 for two different values of voltage VGS1, noted VGS1xe2x80x2 and VGS1xe2x80x3. MOS transistor 18 being of type P, the gate-source voltage is negative and VGS1xe2x80x2 is smaller than VGS1xe2x80x3.
Each curve 35A, 35B includes a first region 36A, 36B, called the linear or ohmic region, in which drain current ID1 strongly varies, substantially proportionally to drain-source voltage VDS1. The equivalent resistance of the transistor between the drain and the source in the linear region is called ron. Resistor ron corresponds to the inverse of the slope of curve 36A, 36B in the linear region. Resistance ron varies according to VGS1.
Each curve 35A and 35B includes a second region 37A, 37B, called the saturated region or constant drain current region, in which drain current ID1 varies very slightly and substantially proportionally to drain-source voltage VDS1. The equivalent resistance between the drain and the source in the saturated region is called R0. Resistance R0 corresponds to the inverse of the slope of curve 35A, 35B in saturated region 37A, 37B. Resistance R0 varies little according to VGS1.
Conventionally, resistance R0 is much higher than resistance ron. As an example, ron may be on the order of 10 ohms and R0 may be on the order of from some ten to some hundred kiloohms.
In a normal operating mode of amplifier 10, output transistor 18 is in the saturated region. In this case, the value of resistance rds appearing in the expression of cut-off frequency f2 is equal to resistance R0. Term gm3*(RL//rds) is then very large as compared to one, and the simplified expression of cut-off frequency f2, given by equation (3), is valid.
However, in some operating modes of amplifier 10, in particular for high values of drain current ID1 (which corresponds to negative voltages VGS1 of high absolute values), output transistor 18 may switch from the saturated state to the linear state.
To illustrate this phenomenon, a load line 39 for a given load resistance has been shown in FIG. 3. The operating point of the circuit formed by the amplifier and the load corresponds to an interception between curve 39 and the curve representative of drain current ID1 according to drain-source voltage VDS1.
It can also be noted that, when the gate-source voltage is equal to VGS1xe2x80x2, operating point 40B is located in saturated portion 37B of curve 35B. In this case, resistance rds is equal to R0 and the simplified expression, given by equation (2), of cut-off frequency f2 is valid.
When gate-source voltage VGS1 is equal to voltage VGS1xe2x80x2, operating point 40A of the circuit is located in linear portion 36A of curve 35A. In this case, resistance rds is equal to ron, which is much smaller than R0. In this case, term gm3*(RL//rds) can no longer be much greater than one, and the value of cut-off frequency f2 associated with second stage 16 decreases.
However, the value of frequency PGB remains substantially constant. As a result, the value of cut-off frequency f2 comes closer to the value of frequency PGB. The condition of steadiness of amplifier 10, which prescribes maintaining cut-off frequency f2 greater than twice frequency PGB, can no longer be fulfilled. An unsteadiness of the amplifier may then occur.
An embodiment of the present invention aims at obtaining an amplifying circuit having its output stage formed by a MOS transistor and in which any unsteady operation of the amplifier is prevented.
For this purpose, an amplifier is provided including first, second, and third series-connected stages, the third stage including a MOS output transistor having its source or drain forming an output terminal of the amplifier, the amplifier further including a first capacitance between the output terminal and the input of the second stage, and a second capacitance between the output terminal and the output of the second stage, and further including means for detecting the transition from a first operating state of the output transistor in which the drain current varies little with the voltage between the drain and the source to a second state in which the drain current varies substantially proportionally to the voltage between the drain and the source; and means for, upon detection of such a transition, having the voltage gain of the amplifier and/or the product between the bandwidth of the amplifier and the voltage gain of the amplifier at the upper limit frequency of the bandwidth drop.
According to an embodiment of the present invention, the means for detecting the transition from the first operating state of the output transistor to the second operating state, and the means for having the voltage gain of the amplifier and/or the product between the bandwidth of the amplifier and the voltage gain of the amplifier at the upper limit frequency of the bandwidth drop comprises an additional MOS transistor.
According to an embodiment of the present invention, the additional MOS transistor is connected as a diode between the gate of the output transistor and the output terminal.
According to an embodiment of the present invention, the additional MOS transistor and the output transistor have a channel of the same polarity type.
An embodiment of the present invention also provides a method for protecting an amplifier including first, second, and third series-connected stages, the third stage including a MOS output transistor, having its source or its drain forming an output terminal of the amplifier, the amplifier further including a first capacitor between the output terminal and the input of the second stage, and a second capacitor between the output terminal and the output of the second stage, including the steps of detecting the transition from a first operating state of the output transistor in which the drain current varies little with the voltage between the drain and the source to a second state in which the drain current varies substantially proportionally to the voltage between the drain and the source; and having, upon detection of such a transition, the voltage gain of the amplifier and/or the product between the bandwidth of the amplifier and the voltage gain of the amplifier at the upper limit frequency of the bandwidth drop.
According to an embodiment of the present invention, the transition from the first operating state of the output transistor to the second operating state switches an additional MOS transistor from an off state to an on state to have the equivalent resistance of the amplifier as seen from the output of the second stage drop.
The foregoing features of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.